Cosmologists have long assumed the universe expands the same way in every direction. A new reanalysis of Type Ia supernova data raises questions about that assumption and about the evidence for a uniformly acting dark energy.
What the Team Reexamined
Theoretical physicist Subir Sarkar (University of Oxford) together with Animesh Sah and Mohamed Rameez (Tata Institute of Fundamental Research) revisited the Pantheon+ catalog of Type Ia supernovae. They applied an age-dependent correction for progenitor stars developed by researchers at Yonsei University and found a systematic trend: after standardization, supernovae from younger progenitors are fainter than those from older progenitors.
How That Affects Cosmic Acceleration
Because more distant supernovae tend to come from younger stellar populations, this systematic faintness makes distant events look farther away than they really are. That extra dimming can mimic the signature of accelerating expansion — the very effect that led to the inference of dark energy.
Anisotropy And A Preferred Direction
The authors report that the apparent acceleration is not uniform across the sky. Instead, it displays a dipole — a preferred direction roughly aligned with our local motion as seen in the cosmic microwave background (CMB) hotspot. The signal also weakens with distance. As the authors put it in Monthly Notices of the Royal Astronomical Society,
"It has been suggested that the cosmic acceleration inferred from Type Ia supernovae could be illusory, due to our being 'tilted' observers embedded in a bulk [cosmic] flow. The inferred acceleration should then be directed mainly along the local bulk flow."
Implications For Dark Energy And Cosmology
If confirmed, these findings challenge a core assumption of standard cosmology: isotropic accelerated expansion driven by a cosmological constant (Λ). A cosmological constant should produce the same effect in every direction and at all distances. A directional, distance-dependent signal is easier to reconcile with local velocity flows or uncorrected astrophysical systematics than with a universal dark-energy term.
These results add to existing tensions in cosmology — notably the Hubble tension, the mismatch between local and early-universe measurements of the expansion rate — but they do not on their own overthrow ΛCDM. The claim is important and provocative, yet it requires independent confirmation and careful scrutiny of possible systematic errors.
Next Steps
The community will test this result by applying the progenitor-age correction to other supernova samples, improving distance indicators, and comparing constraints from independent probes such as baryon acoustic oscillations, the cosmic microwave background, galaxy surveys, and standard sirens from gravitational waves. Larger, more uniform supernova datasets and further theoretical work on progenitor populations will be crucial.
Authors and Source: Subir Sarkar, Animesh Sah, and Mohamed Rameez; study published in Monthly Notices of the Royal Astronomical Society. The analysis uses the Pantheon+ supernova catalog and a progenitor-age correction from Yonsei University.
Note: The results are significant but not yet definitive. Independent reanalyses and new data will determine whether the apparent acceleration is an observational artifact or a genuine cosmological signal.
